1. Field of the Invention
The present invention relates to an image transmission apparatus that transmits image data with variable quality.
2. Discussion of the Background Art
A speed of a communication through a network has been improved on recently. The speed enables to control and to display an image sent from a remote computer over a network in real-time. In such a real-time network computing, full screen images or partial images of a full screen image are compressed and transmitted to a remote device.
For example, a network projector system is available. In such a system, a projector displays an image in real-time sent from a remote personal computer through a wireless LAN, based on standards such as IEEE802.11b and IEEE802.11g.
Additionally, VNC (Virtual Network Computing) is an application to enable such a real-time network computing. The VNC employs a kind of a lossless compression method. Therefore, all images are transmitted without distortion. But, a compression ratio cannot be changed in the compression method. Thereby, a size of the compressed image data sometimes becomes wider than a usable transmission bandwidth. Then, a refresh rate of the image data must be lower than usual.
Japanese patent publication (Kokai) No. 2004-86550 discloses an example of a solution to such a problem. As disclosed in this publication, an image is divided into a plurality of blocks. A block that has changed many times is given a lower priority to send to a display terminal.
However, in this technique, a block that changes frequently still needs a wide transmission bandwidth to maintain smoothness of the image.
Movie compression methods such as MPEG (Moving Picture Experts Group) standards are another example of a solution of the above problem. According to MPEG standards, a difference of an image from a previous appearance of the image is compressed and sent. The previous appearance of the image is extended from a previous compressed data.
In the compression, a bigger quantization unit is used to reduce the compressed data when the amount of the compressed difference data is larger than a usable transmission bandwidth, and a smaller quantization unit is used when the amount of the compressed difference data is smaller than a usable transmission bandwidth.
This method keeps a high refresh rate of the image data, and enables to send a higher resolution image when the difference is little. But an extension processing to obtain the previous appearance of the image is required. The extension process is very taxing.